The present invention relates to an electric machine,                wherein the electric machine has a base body in which a stator of the electric machine is arranged,        wherein the electric machine has a rotor shaft which is rotatably mounted in the base body by way of bearings,        wherein the electric machine has a pick-up element which is connected to the base body in a mechanically rotationally fixed manner and which taps a voltage occurring in the rotor shaft.        
The present invention furthermore relates to a rail vehicle,                wherein the rail vehicle has at least one wheel axle and at least one electric machine,        wherein the at least one wheel axle is connected to a rotor shaft of the at least one electric machine directly or by way of a gear train assembly.        
The present invention further relates to a rail-mounted car set which has a sequence of rail vehicles coupled to one another.
The above-cited subject matters are generally known. With regard to the electric machine, reference is made purely by way of example to DE 10 2007 019 828 B3 and U.S. Pat. No. 7 193 836 B2.
In electric motors, shaft voltages can occur in the rotor shaft, thereby inducing bearing currents. The bearing currents are essentially undesirable, but not really disruptive in every case. The most serious type of bearing currents are what are referred to as EDM bearing currents (EDM=electric discharge machining), because bearing currents of said type very quickly lead to bearing damage. With bearing currents of said type an electric arc is produced between the rollers and the race of the bearings. As a result thereof the lubricating grease burns and craters are produced in the bearing shells.
In addition to EDM bearing currents, capacitive bearing currents, conventional shaft voltages, high-frequency circular current and capacitive rotor ground currents can also occur.
In order to eliminate and suppress bearing currents it is known in the prior art to provide the electric machine with a pick-up element which is connected to the base body in a mechanically rotationally fixed manner and which taps a voltage occurring in the rotor shaft. The prior art cited in the introduction reveals embodiments of said type.
However, this approach is problematic, in particular in the case of rail vehicles. The reason for this is that the pick-up element causes an electrically conductive connection to exist from the rail by way of the wheel axle and possibly a gear train assembly to the motor housing. The connection can exhibit a very low impedance. As a result it can happen that a considerable amount of an operating current which is injected by another rail vehicle (in particular a rail vehicle of a different train) as a return current into the rail flows through the rail vehicle. This can cause interference to the communications equipment on the rail vehicle or the vehicle grounding system, in particular the shield of data lines, can be overloaded. This danger is present especially in the case of long vehicles having car bodies made of aluminum, since as a result the vehicle makes such a large conducting cross-section available that the specific resistance per meter of car length is less than the specific resistance per meter of rail. For this reason tapping devices of the above-cited type are not used in rail vehicles in most cases.